1. Field of the Invention
This invention relates to a fluid control device and a method of manufacturing the same. More particularly, the present invention relates to a fluid control device applicable to filters, flow rate control systems and separators to be used for gas and fluid.
2. Related Background Art
Filter membranes are typical examples of known fluid control devices. Organic filter membranes of polymer materials including cellulose based, polysulfone based and polyolefin based polymer materials are being popularly used. Inorganic filter membranes of porous materials including Vycor glass, carbon tubes, silica based ceramic materials and anodized alumina film are also popularly being used. Films of the above listed organic materials are also being used as ultrafilter membranes.
Filter membranes as listed above are semipermeable films mainly having very fine pores of several nanometers to tens of several nanometers and adapted to operate on the basis of utilization of the pressure difference between the front side and the rear side, or the upstream side and the downstream side, of the film as drive force.
Anodized alumina film that is closely related to the present invention will be described in greater detail below.
Anodized alumina film is a porous film in which fine pores grow from the film surfaces in a direction perpendicular to the surfaces. The porous oxide film is characterized in that it has a peculiar geographical structure in which very fine pores having a diameter (2r) of several to hundreds of several nanometers are arranged substantially in parallel with each other with a gap (2R) of tens to hundreds of several nanometers separating adjacent ones. The pores show a high aspect ratio and their diameters are highly uniform. It is possible to control the diameter 2r and the gap 2R to a certain extent by regulating the electric current and the voltage that are used in the anodization process.
When anodized film of aluminum is used for a filter membrane, it needs to be peeled off from the underlying aluminum.
Japanese Patent Application Laid-Open Publication No. H4-12169 discloses a method of forming a porous film by anodizing metal aluminum or an aluminum alloy and subsequently removing all the metal by etching except the porous part thereof.
Japanese Patent Application Laid-Open Publication No. H7-112531 discloses a method of carrying an anodized film by means of pits produced by electrolytic etching, which does not involve the technique of removing the underlying metal by etching, using a solution.
Japanese Patent Application Laid-Open No. H2-218422 discloses a method of manufacturing a film, in which large diameter pores and small diameter pores are linked by modifying the anodizing condition in the course of anodization process.
U.S. Pat. No. 6,279,913 discloses a fluid control device using an anodized film of an aluminum alloy to which silicon is added. While a material of ADC12, AC4C or A4000 type is used as aluminum alloy to which silicon is added, silicon is not uniformly dispersed in such a material and hence it can deter the process of anodization.
The above listed known fluid control devices, organic filter membranes in particular, have drawbacks in terms of chemical-resistance and heat-resistance. On the other hand, it is difficult to mold films of inorganic materials, in particular those of ceramic materials.
Anodized alumina film is an inorganic material that is highly resistant relative to alkaline aqueous solutions, acidic solutions and organic solvents, although strong alkaline solutions are exceptions. While it is formed as film whose profile can be controlled to some extent, pores are apt to show a diameter greater than 10 nm because an etching operation is conducted in the step of peeling off the alumina film.
When anodized alumina film is used for filters, not only the pore diameter but also the flux (the flow rate per unit area) of the film is important. As the pore diameter is reduced or the pore length is increased, the flux decreases proportionally. In order to increase the flux and reduce the pore diameter, the thickness of the porous film having a small pore diameter needs to be reduced. However, it is difficult to prepare such a film.
It is also very difficult to control fine pores having a diameter smaller than 10 nm so as to make them show a uniform diameter.
Thus, it is an object of the present invention to dissolve the above identified problems and provide a film of an inorganic material having fine pores with a diameter not greater than 10 nm. In view of the fact that a porous film having a pore diameter not greater than 10 nm can show a low flux when it has a large thickness, it is another object of the present invention is to provide a carrier film that can be used to carry a porous film on a stable basis when such a porous film is prepared with a reduced thickness.
Still another object of the present invention is to provide a method of manufacturing a porous film.